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Non-fungal graystain is a common defect in yellow-poplar lumber. Fumigation of fresh logs with a fumigant (sulfuryl fluoride) known to kill living parenchyma cells throughout the sapwood was successful in prevention of graystain in lumber after air-drying and planing. However, increased fungal blue stain was noted in fumigated materials as compared to controls, which stresses the need for fungicide protection during seasoning.

Many, if not most of the important hardwood lumber species are subject to discoloration in the sapwood by non-microbial staining often called "graystain" or "enzyme-mediated" oxidative stain [2]. Recently, researchers have documented the ability to prevent such staining by killing the parenchyma cells in fresh logs and presumably preventing the accumulation of enzymes and/or stain precursor chemicals in the sapwood [1, 6]. This approach has proven successful for a variety of hardwood species using methyl bromide or sulfuryl fluoride (SF as the fumigant [3,5,7,8]. However, no data have been provided for graystain prevention in yellow-poplar (Liriodendron tulipifera L.). This white wood species is increasingly being used where aesthetic appearance of sapwood is desired (e.g., millwork interiors), and staining can greatly decrease value for such applications.

The objective of this study was to determine whether SF (preferred over methyl bromide because it causes no ozone layer destruction) fumigation of logs of yellow-poplar could prevent graystain when logs were processed according to a commercial mill operation. In addition, the effect of log end-coating with a fungicide and water repellent on development of fungal stain during log decking was assessed. Finally, colorimetric readings of graystain, clear wood, and fungal-stained portions of lumber were documented.


Two yellow-poplar trees were cut in August from the Darlington Woods experimental forest of Purdue University. Maximum diameters were 370 and 340 mm with maximum sapwood thick-nesses of 90 and 80 mm, respectively. Each log was cut into three bolts (each 2.7 m long, and one bolt from each log was placed into one of three treatment piles: Controls, SF (low dose, and SF (higher dose. Prior to fumigation, one end of each log was coated with a commercial fungicide (Busan 1067 with TCMTB and MBT as the active ingredients; Buckman Laboratories, Memphis, Tennessee followed by a wax application (Anchorseal, UC Coatings Corp., Buffalo, New York. This end treatment was done to assess its influence on fun gal entry into log ends during the decking period prior to sawing of lumber. Bolt stacks were covered with a gas-proof tarp and the two treatment piles were given target gas loadings of 250 and 375 g of SF per [m.sup.3] of space under the tarp (these levels had been successful in treatments of other hardwood species). B olts (felled 7 days prior) were fumigated for 72 hours with gas concentrations recorded three times daily using a thermal conductivity analyzer (Fumiscope Model D, Key Chemical and Equipment, Clearwater, Florida). These data were used to calculate the concentration x time (CT) product as an overall measure of treatment commonly used for assessing fumigations of wood materials.

Effectiveness of the SF treatments in killing the living parenchyma cells in log sapwood was determined by taking an increment core (from bark to heartwood) from the center of each bolt and soaking them in a I percent solution of tri-phenyl tetrazolium chloride (TTC). A red color develops in the cores if parenchyma cells are living (retain dehydrogenase activity [4]).

Bolts were then stored in the sprinkler yard under intermittent water spray to discourage fungal and insect attack for 6.5 weeks. Such wet-deck storage has been suggested to increase the propensity for graystain as parenchyma cells remain viable in unfumigated materials. Lumber was then cut (nominal 25 mm thick) from stored logs with staining by fungi from log ends noted by treatment. Green lumber was solid stacked for 4 days and then stickered and placed in the air-drying yard for 3 months (no fungicide dip) before planing and evaluation of stain in sapwood.

Two representative board pieces with clear, graystain, or fungal stain were examined using a colorimeter (Minolta Chroma Meter model CS-l00, Minolta Camera Co., Ltd., Japan) to note how each color would vary in stained areas and, on average, be represented using L x [alpha] (Lightness x red) coordinates. Ten areas from each board were measured under indoor lighting for each type of surface appearance (n = 20 for each average).


The CT products for the SF treatment piles were calculated to be 25,700 and 32,570 g-hr./[m.sup.3] for the lower and higher treatment dosages. The TTC assay on cores showed that both treatment levels were sufficient to kill all sapwood parenchyma cells (no color developed, as compared to the pink to red color that developed in the sapwood of the controls, as deep as 30 to 60 mm from the bark).

Observations of fungal entry into bolt ends after the wet-deck period were mixed as to the effect of fumigation on promotion of sapstain. Both control bolts had little or no fungal staining into ends whether end-coated or not. One yellow-poplar log (one bolt at each SF treatment level) likewise was essentially free of end stain by fungi even on the Uncoated end; however, the second yellow-poplar log bolts were obviously invaded from 130 to 230 mm by blue stain fungi only on the untreated ends. These data, though from limited sample numbers, suggest that the intermittent wetting in the deck was generally successful in prevention of log end staining by fungi, but if logs were not so protected (by position in the deck stack relative to sprinkling distribution), fungal entry into fumigated logs could be substantial within the 6.5-week decking period. However, the end-coating was effective in these fumigated bolts in the prevention of blue stain.

Graystain evaluations of dry and surfaced lumber noted that 45 percent (10 of 22) of the boards from control bolts had obvious (slight to moderate) grayStain on at least one face. No graystain was noted on any of the 46 total boards from the two SF treatment bolts. The fumigation treatment was successful in prevention of graystain. As these boards had not received any fungicide dip prior to air-drying, fungal stain was noted in most of the boards after planing. The non-fumigated materials were generally only lightly stained by fungi (less than 5 percent of sapwood discolored), but the majority of boards from fumigated bolts had moderate to heavy fungal stain.

These observations follow earlier findings that killing of parenchyma does promote more vigorous fungal attack of wood during storage and air-drying [5,7], and that measures to minimize fungal discolorations must be taken to preserve lumber quality.

Average colorimeter values for graystain in yellow-poplar showed generally only a drop in lightness (L) value (from 70 to 60) with only a very small shift on the red (a) scale (Fig. 1). These rather small differences and the variation encountered for each condition (graystain seen by the eye as noticeable as compared to clear sapwood) might make automated selection of graystain in yellow-poplar difficult by instrumental color measurement only. However, the blue stain regions are not only darker (lower L value), but record much lower red values, which should make automated defect detection easier.

The authors are, respectively, Professor, Dept. of Wood and Paper Sci., Univ. of Minnesota (UM, 2004 Folwell Ave., St. Paul, MN 55108; Professor, Dept. of Forestry and Natural Resources, Purdue Univ., 1200 Forest Products Building, West Lafayette, IN 47907; and Research Assistant, Dept. of Wood and Paper Sci., UM. The essential help in all phases of this work provided by C.C. Cook Lumber Co., Reelsville, IN, is gratefully acknowledged as is on-site technical assistance and financial support by Dow Agroscience, Indianapolis, IN, as provided through the MN Sota Tec Fund. This paper was received for publication in July 2000. Reprint No. 9149.

(*.) Forest Products Society Member.

[c]Forest Products Society 2001.

Forest Prod. 3. 51(9):50-52.


(1.) Amburgey, T.L., E.L. Schmidt, and M.G. Sanders. 1996. Trials of three fumigants to prevent enzyme stain in lumber cut from water-stored hardwood logs. Forest Prod. J. 46(11/12):54-56.

(2.) Forsyth, P.G. and T.L. Amburgey. 1992. Prevention of non-microbial sapstains in southern hardwoods. Forest Prod. J. 42(3): 35-40.

(3.) Kreber, B., E.L. Schmidt, and T. Byrne. 1994. Methyl bromide fumigation to control non-microbial discolorations in western hemlock and red alder. Forest Prod. J. 44(10):63-67.

(4.) Ruetze, M. and W. Liese. 1985. A post fumigation test (TTC) for oak logs. Holzforschung 39:327-330.

(5.) Schmidt, E.L., D.L. Cassens, and J. Steen. 1997. Log fumigation prevents sticker stain and enzyme-mediated sapwood discolorations in maple and hickory lumber. Forest Prod. J. 47(9):47-50.

(6.) _____ and E.R. Christopherson. 1997. Effects of fumigants on parenchyma viability in red oak log sections. Forest Prod. J. 47(5):61-63.

(7.) _____, _____, and S.C. Kitchens. 1998. Mill trial confirms control of lumber graystain and sticker shadow after fumigation of hardwood logs with methyl bromide. Forest Prod. J. 48(6):50-52.

(8.) _____, T.L. Amburgey, M.G. Sanders, and C.D Bell. 1998. Sulfuryl fluoride and methyl bromide fumigation of hardwood logs of various ages for control of lumber graystain. Forest Prod. J. 48(11/12):77-80.
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Publication:Forest Products Journal
Geographic Code:1USA
Date:Sep 1, 2001

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